It is a well-known fact that with the advent of fiber-based Packet Over SONET (POS) services, Asynchronous Transfer Mode (ATM) is losing its importance in the WAN environment. ATM is already considered an outmoded technology in the Local Area Network (LAN) environment. Several networking vendors use POS in lieu of ATM in the Wide Area Networks (WANs). Engineers today believe that large networks must not waste 20 to 33% of the bandwidth for a limited set of capabilities such as Constant Bit Rate (CBR) from ATM. Additionally, some engineers consider ATM Segmentation and Re-assembly (SAR) process to be non-scalable at or above 2.4 Gbps. In case of nonstandard ATM devices, even virtual circuit merging process causes scalability problems. The missing requirement in all these deployments is the guaranteed QoS. Missing also are features such as enhanced security, localized failure recovery, congestion avoidance and robust network management. The latter can for instance include billing support facility. ATM has failed to provide these features. If ATM is not the vehicle, then POS being an Internet Protocol (IP) service becomes the next choice. But POS can at best provide a coarse Class of Service (COS) by using IP header's Type of Service (TOS) bits and applying the common routing protocols. With no direct content awareness imbedded in the network, there is thus little or no end-to-end QoS one can expect from POS.
This then brings us to Multiprotocol Label Switching (MPLS). But as people are finding out, MPLS is no panacea either. The most it provides today for QoS is COS from the IP TOS bits. MPLS has no built-in framework for network-imbedded QoS services such as Constant Bit Rate (CBR), Variable Bit Rate (VBR), Real-Time VBR (VBRrt), or others. MPLS also does not allocate any means of deploying failure-proof, TCP-based management features in the network. It does not provide any built-in means for local failure recovery or congestion avoidance. All these requirements bring back ATM as the Layer 2 mechanism for MPLS. But like ATM, MPLS wastes a lot of network resources and adds to operational costs for its label distribution processes as well as in processing variable sized packets due to variable number of labels that are attached to the IP packets from node to node.
Finally, MPLS does not add any strength to security or network management needs. These needs are growing astronomically with the growth of the Internet.